The present invention relates to molds for cooling, solidification and casting a molten metal that is supplied, so as to allow continuous casting of a metal such as steel, and therefore relates in particular, to adjustable molds of horizontal continuous casting apparatus that can perform uniform cooling of castings.
Continuous casting is performed by supplying a molten metal stored in a tundish to a mold where at least the outer portion is cooled and solidified to form a casting, which is continuously taken by an extraction apparatus provided on the downstream side of the mold. The mold that is used in this continuous casting is generally formed in a cylindrical shape and the outer peripheral surface is cooled (generally, by water) so that the molten metal that is supplied to the cavity inside it is solidified to form the castings. The mold comprises a material that has an excellent heat conductivity for the cavity portion that corresponds to the shape and dimensions of the section of the required casting. The mold is also configured with a cooling water jacket provided to the outer side of the mold so that cooling water can flow along the outer peripheral wall.
Accordingly, the heat of the molten metal is removed by this cooling water and the molten metal solidifies to form the casting. The casting direction (the mold axis in the direction of the length) for continuous casting that is performed in this manner need not be vertical, but can be set to be horizontal or at a slope, while the section of the cavity portion can be rectangular, polygonal, circular or some other shape.
There are the following two major types of mold.
(a) Molds for monolithic formation of cylindrical bodies
Tubular molds have either a single circular or an angular section and are used for obtaining billets of small sectional area. Rectangular billets having a large area (slabs, blooms) are obtained from assembled molds that have a plural number of casting elements formed by the periphery of the casting being divided in the direction of the casting section, tightly joined and assembled to form a cylindrical body. However, either type has a section where the internal peripheral wall of the cavity portion is continuously closed, and are used as fixed casting molds for which the sectional dimensions of the cavity portion do not change during casting.
The billets shrink and their sectional dimensions decrease when they are cooled and solidify. With molds that continuously cast such cylindrical bodies, maintaining contact between the mold and the billet involves forming a suitable taper to the inner peripheral wall of the mold so that the dimension becomes smaller on the downstream side. However, the shrinkage ratio of the billet differs because of many factors that include the type of metal being cast, the inlet temperature and the casting extraction speed and the like.
Because of this, it is difficult to maintain a uniform contact between the inner peripheral wall and the surface of the billet by simply forming a taper on the inner peripheral wall of the casting. One common means of eliminating this problem is to shorten the stationary mold and to provide an adjustable mold, described later, on the downstream side.
(b) Molds forming cylindrical bodies by a plural number of mutually separable mold elements
There are also molds known as adjustable molds that have a plural number of adjustable mold elements arranged in the direction of the radius of the casting. These molds are used as adjustable molds that have the sectional dimension of the cavity portion changing during casting. Each of the elements of these adjustable molds are arranged so that they do not come into contact in the direction of the periphery of the sectional surface of the casting and are pressed into the billet surface by an urging means such as a spring or a hydraulic cylinder. Gaps are provided between each of the elements of the adjustable mold so as to enable this movement and these gaps are provided at a position after the suitable formation of a solidified layer on the surface of the molten metal, that is, at a position on the downstream side of the stationary mold.
As has been described above, the billet shrinks along with the progress of cooling and solidification. However, in such an adjustable mold, each of the mold elements is pressed into the surface of the billet and so there is a favorable contact with the billet surface and when compared to stationary molds, it is possible to have more uniform cooling.
One combination of such a stationary mold and an adjustable mold is disclosed in Japanese Patent Laid-Open Application No. 32104-1990 (hereinafter termed "conventional technology".) As shown in FIG. 13 and FIG. 14, this conventional technology has a first mold portion 241 (equivalent to the stationary mold), and second mold portions 242,243 that are arranged on the downstream side of the first mold portion 241 (and which are equivalent to the adjustable mold), and wall portions 244 (equivalent to cooling plates) that are the second mold portion divided into four respective portions in the direction of the peripheral surface of the casting. These wall portions 244 are configured so as to be movable in the direction of the radius of the section of the casting by an adjustment means 245 (such as a reciprocating hydraulic cylinder, for example) that is arranged in a direction parallel to the direction of casting.
Furthermore, the adjustment means 245 connected to a outlet portion 248 and a inlet portion 247 of the wall portion 244 by a bell crank 246 that converts the direction of motion of the adjustment means 245 that is parallel with respect to the direction of casting, into the direction (equivalent to the direction of the casting section radius or a vertical direction to the casting direction) which is substantially perpendicular with respect to the direction of casting.
However, there are the following problems when continuous casting is performed using a continuous casting mold according to this conventional technology. More specifically, there is the problem of deformation and cracking of the billet when there is uneven cooling of the billet, and the problem that there is not sufficient reliability of operation of the bell crank mechanism.
1) Generation of billet deformation and cracking
(a) The taper that is provided to the stationary mold is set beforehand for each metal to be cast, on the basis of precise calculation and testing. If the amount of this taper of the mold is set larger than the amount of shrinkage of the billet, then the smooth extraction of the billet will not be possible. Conversely, if this amount is set small, then there will be a gap between the billet and the mold and the transfer of heat will be prevented, and there will be no progress of billet cooling. PA1 (b) In addition, since control of the pressing force of the adjustable mold is not performed, there is an abnormal increase in the force of friction between the adjustable mold and the solidified layer at the surface. Because of this, the billet is crushed and the molten metal inside the billet that is solidifying, overflows or the adjustable mold is pressed back by the static pressure of the molten metal inside the billet that is solidifying. The result of this is that there is insufficient cooling. PA1 (a) One conventional technology as shown in FIG. 14 is a method that operates a wall portion 244 (cooling plate) of an adjustable mold by using a bell crank 246 to convert the direction of motion of an adjustment apparatus 245 (hydraulic cylinder). Because of this, it is difficult to expect accurate operation in a continuous casting apparatus that is under environmental conditions of high temperature, humidity and dust levels. PA1 (b) A wall portion 244 (cooling plate) is supported by a free connector 249 (ball connector) and the bell crank 246 is linked to the wall portion 244 (cooling plate) by a pin 251 that has a gap 250 for play. Because of this, it is difficult to set an accurate press length.
However, when actual continuous casting is performed, it is rare for the billet to shrink along the taper of the mold. This is to say that the shrinkage ratio of the billet changes according to the temperature of the molten metal and the casting speed and so even if the type or the components of the casting metal are the same, the shrinkage ratio will change for each casting or with the elapse of time during casting. As a result, even as the cooling and solidification progresses, there will be little shrinkage relative to the original sectional figure, and in practically all cases, the sectional figure of the billet will deform to become more elliptical or rhomboid or the like.
As has been described above, the formation of a gap between the billet surface and the mold prevents the transfer of heat. Because of this, if the billet is deformed and there is uneven contact with the mold, then there will be large deviations in the intensity of cooling between the gap portion and the contacting portion. The occurrence of such a distribution of the cooling intensity contracts the billet so that there is promotion of deformation, and so the deformation and the non-uniform cooling increases until the billet leaves the mold. As a result, there is the formation of either cracking or a non-uniform or an asymmetrical solidification structure inside the billet.
There are also molds that press mold elements to the billet surface in which the stationary mold is shortened and an adjustable mold is connected downstream so that this progressive deformation and non-uniform cooling does not occur. However, according to a continuous casting mold of the conventional technology, there is no control for the pressing force of the adjustable mold in the direction of the casting section radius and so it is easy for the wall portions 244 (cooling plates) of the mold pressed to the billet to press against portions that are weak, that is, those billet portions (those portions close to the stationary mold) for which the solidified layer of the molten metal surface is weak. As a result of this, the billet is easily deformed and broken.
In cases such as these, as shown in FIG. 13 and FIG. 14, the length of the first mold portion 241 (stationary mold) is short and the thickness of the solidified layer of the billet surface that can be cooled by the first mold portion 241 is thin and so it is easy for the billet to be crushed at the entrance to the adjustable mold.
On the other hand, when the length of the first mold portion 241 (stationary portion) is long, there is considerable progress of deformation of the billet due to non-uniform cooling inside the first mold portion 241. Because of this, it is not possible to expect that billet deformation can be suppressed by the prevention of non-uniform cooling in an adjustable mold.
2) Lowering of operating reliability through use of bell crank mechanism